I'm both curious and undecided on the OP's idea.
Full disclosure before I start rambling: I play a level 14+ character in Barovia. I lead a faction. We have a base. Probably 80 percent of that character's RP has happened exclusively in Barovia. Substantial chunks of it have happened in faction bases. There are a mixture of reasons why I don't move this character to Port long-term (nothing to do with the Port players, of course). I don't mind not getting RP XP in Western Barovia. The story has always seemed more important to me. This character will need a lot of time training elsewhere (time I rarely have IRL) to gain significant experience, so I've accepted that he probably won't advance much further in level (if at all) for the remainder of his time on the server. But that's just me.
With that out of the way, I'm curious what areas will qualify as "faction bases" under this proposal. There are the obvious ones, like the Kinship Lodge, the Red Vardo Offices, and the Citadel. What about the Refuge of Fifth Light for the Ezrites? Would the Sanctuary of Coming Dawn be counted as a faction base because that's where so many MLers spend their time, or would it only be the Sanctuary of Eternal Dawn to avoid all players loitering in the Coming Dawn to get RP XP, even when doing so doesn't make sense? Would the Drain count? Would rentals count? If so, how does that work? Would (just making this up) Bob Bobescu the wheelwright and his employees who collectively own Shop I get RP XP for RPing together in their shop with no one else around despite not being in a faction?
I'm not trying to criticize or critique. I'm genuinely intrigued as to what this system would look like. Did you have something specific in mind, Iolantir? Obviously, it's not all on you to come up with. I'm just wondering if you have more thoughts on it.
For me (speaking as the current head of a faction), the proposal boils down to a question of privilege and the mechanics of how to be more egalitarian. The proposal obviously rewards involvement in factions, which makes some sense: faction bases are RP hubs, and factions (when staffed well and working at peak performance) help to drive RP and plots for their members and the larger community.
However, as another poster points out, there's the concern of faction members having the privilege of RPing behind these plot-locked doors and getting RP XP for it. On one hand, that incentivizes people to either join or interact with these factions. Those are both welcome things. On the other hand, during the many times it's not practical or wise to simply throw open the doors and invite tons of people in, it creates a scenario where faction members are getting RP XP while a large portion of the Vallaki community is shut out from getting it because the nature of the discussion or event (clandestine meetings, plotting against Strahd, planning an assassination, etc.) in the faction base excludes participation from external characters. That's kind of a disadvantage.
There's also the concern that there are many large and/or legitimate factions who simply don't have a faction base (because all the rentals are taken) or can't afford one. The Crows, for instance, have had a good presence recently but don't have a faction base. The Gundarakite Rebels are another. These characters would be disenfranchised from the same RP XP opportunities that other factions who do have bases get, simply because there aren't enough rentals to go around. (The Gundarakites are a special case and wouldn't get a rental in Vallaki, but that's beside the point.) And while these characters could come RP in an existing faction's base, that's not always as simple, easy, or sensible as having your own base.
Another concern is faction base usage. Ideally, this RP XP system would encourage people to RP in faction bases and encourage factions to host events that draw people to the bases. It's sensible to think that there may eventually be an expectation that that's what happens. How is the success of that measured? Will factions that aren't able, for various reasons, to hold large or consistent events for various stretches of time have their faction base taken away, despite using it for smaller, internal-only functions more regularly? I don't want to fatalize, and I can't envision it happening, but it's a possibility with consequences that should be considered.
Altogether, I'm neither for nor opposed to the OP's proposal. I think there's some merit in it, given that level 14+ characters are encouraged to give up direct participation in many things in Barovia in favor of RP, facilitation, and support. I just think some extra scaffolding would need to be built around the idea to make it fair and viable for as many characters and factions as possible.
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Without derailing the points above or the thread as a whole, I've seen a couple of comments about being 14+ in Barovia. I'm not going to get into whether it should or shouldn't be; that's a discussion for another thread. It is what it is for now.
What being 14+ in Barovia really boils down to is "be respectful." That's stuff like trying to avoid sweeping dungeons for loot or fun, working with (A)MPCs' players to drive story rather than put another notch on the monster-hunting belt (even when you're PvPing or "hunting" them), asking (A)MPCs' players if they're comfortable with you initiating some hostile confrontations before doing so, facilitating plots and RP for others, sharing hooks and giving lower-level characters a chance to get involved and shine, and so on. Sometimes it may mean pulling punches even when it doesn't make IC sense. In the end, it's about teamwork.
I think Evendur captured it well with their post above. High-level characters can enhance ambiance. Being respectful is key. ("Being excellent," even?)
I'm not going to pretend for a second like I've always gotten that right. It just seems to me that it's something we all (myself included) can always bear in mind. That's just my two cents on it.